Through the act of breathing, thoracic and abdominal anatomy is in constant motion and is typically irregular. This irregular motion can exacerbate errors in radiation therapy, breathing guidance interventions operate to minimise these errors. However, much of the breathing guidance investigations have not directly quantified the impact of regular breathing on radiation therapy accuracy. The first aim of this thesis was to critically appraise the literature in terms of the use of breathing guidance interventions via systematic review. This review found that 21 of the 27 identified studies yielded significant improvements from the use of breathing guidance. None of the studies were randomised and no studies quantified the impact on 4DCT image quality. The second aim of this thesis was to quantify the impact of audiovisual biofeedback breathing guidance on 4DCT. This study utilised data from an MRI study to program the motion of a digital phantom prior to then simulating 4DCT imaging. Audiovisual biofeedback demonstrated to significantly improved 4DCT image quality over free breathing. The third aim of this thesis was to assess the impact of audiovisual biofeedback on liver cancer patient breathing over a course of stereotactic body radiation therapy (SBRT). The findings of this study demonstrated the effectiveness of audiovisual biofeedback in producing consistent interfraction respiratory motion over a course of SBRT. The fourth aim of this thesis was to design and implement a phase II clinical trial investigating the use and impact of audiovisual biofeedback in lung cancer radiation therapy. The findings of a retrospective analysis were utilised to design and determine the statistics of the most comprehensive breathing guidance study to date: a randomised, stratified, multi-site, phase II clinical trial.. The fifth aim of this thesis was to explore the next stages of audiovisual biofeedback in terms of translating evidence into broader clinical use through commercialisation. This aim was achieved by investigating the the product-market fit of the audiovisual biofeedback technology. The culmination of these findings demonstrates the clinical benefit of the audiovisual biofeedback respiratory guidance system and the possibility to make breathing guidance systems more widely available to patients
